As a result of enforcement of Construction Material Recycling Law (Law Concerning Recycling of Materials from Construction Work [Law No. 104 of May 31, 2000]), concrete blocks, asphalt concrete blocks and lumber originating from construction work are required to be sorted and recycled.
The Kyoto Protocol was voted in 1997 according to the United Nations Framework Convention on Climate Change and the target reduction ratios of the developed countries were determined relative to the reference emission levels of 1990 for carbon dioxide, methane, nitrogen monoxide, HFCs and sulfur hexafluoride. The target values are to be achieved by the jointly set time limit. The Kyoto Protocol was ratified by the Japanese Diet on May 31, 2002 and Japan deposited the instrument of acceptance in the United Nations on Jun. 4, 2004.
The determined target reduction ratios from the year of 2008 to the year of 2012 include 6% for Japan, 7% for United States of America and 8% for the European Union.
The present invention is intended to respond to the above-identified demand of the international society.
As for lumber originating from construction work, the target recycling ratio is set to 95% including reduced (incinerated) wood. Such wood is currently recycled after being crushed to wood chips in order to produce boards for particles, raw materials for paper producing, compost and so on and also for applications including mulching, thermal recycling (fuel) and chemical recycling. However, improper deposits (illegal dumping) of wood waste are frequently observed and fires have sometimes broken out as a result of spontaneous ignition of wood waste.
Wood is the most popular material that is being used in houses. However, many wood plates including those used for repairing are cross-laminated plates prepared by using paint and adhesive and hence can hardly be recycled to produce materials. In other words, most of them are thermally recycled (as fuel).
Lumber originating from construction work amounts to 9,000,000 tons per year only in the Kanto District in Japan. However, if wood waste is not reduced (incinerated) by thermal recycling but utilized as raw material for paper producing, a forest area that is two to three times as large as the forest area that needs to be destroyed to produce wood can be conserved on this planet because paper can be recycled two to three times. Then, the effect of supplying oxygen and suppressing carbon dioxide gas can be doubled or tripled.
While lumber originating from construction work is advantages when it is recycled as raw material for paper producing, the Kraft process that currently takes about 95% of the overall pulp production consumes high-quality wood chips as raw material for paper producing. In other words, lumber originating from construction work takes only a fraction of the overall pulp production in the world.
Currently, the Kraft process takes about 95% of the overall pulp production in the world, using wood chips as principal raw material. With this producing process, water, caustic soda and sodium sulfide are added to wood chips mainly obtained by crushing lumber cut out from woods to make them show a certain ratio and the materials are cooked (boiled) in a vessel at an average temperature of 160° C. under 6 to 7 atm for not less than 3 hours. Then, lignin that operates to bond the cellulose contained in the wood chips is continuously dissolved into the solution to isolate cellulose and hemicellulose from each other and produce pulp as lump of cellulose. The obtained pulp is generally referred to as fresh pulp. This process is the best pulp producing process in terms of cost performance at present.
Now, the Kraft process will be summarily described below by referring to FIG. 1 of the accompanying drawings.
Chips are sorted to make them show a thickness and a length that are found respectively within certain ranges and the dust is removed from them so that they may be cooked and digested uniformly.
In the Kraft digesting step, water and chemicals (caustic soda and sodium sulfide) are added to the chips to make them show a certain ratio and the materials are cooked (boiled) at 160° C. under 6 atm for not less than 3 hours to dissolve lignin.
In the subsequent washing step, the solution of the chemicals (caustic soda and sodium sulfide) containing the lignin dissolved in it (black liquor) is separated from the pulp, which is then washed with water.
In the delignification step, the lignin remaining in the pulp is eluted by means of oxygen and alkali.
In the filtering/refining step, the foreign materials such as dirt contained in the pulp is isolated and removed.
In the bleaching step, the pulp is bleached by means of a chemical selected from chlorine, chlorine dioxide, oxygen, caustic soda and sodium hypochlorite.
On the other hand, the black liquor is concentrated. The concentrated black liquor used as fuel within the process and collected as sodium carbonate.
Some Patent Documents relating to the Kraft process are listed below.    [Patent Document 1] JP 2006-274500-A
Patent Document 1 describes a process of turning green liquor, caustic.    [Patent Document 2] JP 2001-172888-A
Patent Document 2 describes a process of bleaching pulp.    [Patent Document 3] JP 11-286889-A
Patent Document 3 describes a PA process (hydrogen peroxide-alkali process) where hydrogen peroxide, a caustic alkali and a small amount of digestion promoter are used. It tells that hydrogen peroxide shows a delignification effect.    [Patent Document 4] JP 08-188976-A
Patent Document 4 describes that a surfactant and a chelating agent are added when bleaching chemical pulp by ozone.    [Patent Document 5] JP 08-502556-A
Patent Document 5 describes a process of bleaching pulp showing a high viscosity relative to permanganate number.
While the Kraft process provides the advantage of being able to produce high-quality paper, it is accompanied by the problems listed below.
(1) It utilizes wood chips obtained by cutting trees from forests as natural raw material. Many forests are destructed to meet the massive demand for wood chips, entailing a serious environmental problem.
(2) It requires a pressure vessel that can be used for digesting at 160° C. and under about 6 atm in order to elute lignin from wood. Then, a large facility has to be installed at high cost for such an operation. The running cost and the energy consumption rate of such a large facility are also high.(3) Since lignin is dissolved at high temperature under high pressure, hemicellulose and cellulose are eluted partly to make the yield as low as about 50%.(4) Since the digesting solution contains sodium sulfide, hydrogen sulfide, methyl mercaptan, dimethyl sulfide and dimethyl disulfide are produced from the digesting step, requiring anti-nuisance measures for the offensive odor they emit.(5) Since the process consumes a large quantity of high-quality water, it requires an ample source of water and drainage disposal, which are costly.(6) Since the yield of pulp production is about 50%, about 50% of the supplied wood is isolated in black liquor as lignin. Then, carbon dioxide gas is produced at a high rate as a result of condensation and incineration.
(N. B.) Lignin is a carbohydrate like cellulose and contains carbon at a ratio of C(12)/CH2(30)=0.4. Lignin is produced by 1 ton as a result of producing 1 ton of pulp. Then, 0.4 tons of carbon turns to carbon dioxide when the lignin is incinerated.
C:CO2=12:44. Thus, 0.4×44/12−1.47 tons of carbon dioxide are produced from 1 ton of lignin (0.4 tons of carbon).
(7) As clear from above, the process is for producing high-quality pulp exclusively from high-quality wood chips and hence is not related to producing pulp from lumber originating from construction work. Additionally, it produces carbon dioxide gas to a large extent.